In imaging apparatuses such as digital cameras, silicon (Si)-based two-dimensional image sensors such as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) are used as imaging sensors. Such a Si-based imaging sensor is sensitive to light of wavelengths in the infrared region and has wavelength characteristics incompatible with the visual sensitivity of humans. In imaging apparatuses, therefore, a filter (infrared-cut filter) for shielding against incident light of wavelengths in the infrared region is typically disposed ahead of an imaging sensor so that an image obtained by the imaging sensor comes closer to an image as perceived by humans.
For example, Patent Literature 1 describes a composite filter including an absorptive cut filter (light-absorbing element) and a reflective cut coating (interference film) provided on a surface of the absorptive cut filter.
Patent Literature 2 describes an infrared-cut filter made up of an infrared absorber and infrared reflector which are bonded together. The infrared absorber is produced by forming an anti-reflection film (AR coating) on a principal surface of an infrared-absorbing glass. The infrared-absorbing glass is a blue glass in which a coloring matter such as copper ion is dispersed. The anti-reflection film is formed by using a vacuum deposition apparatus to vacuum-deposit a single layer film made of MgF2, multilayer film made of Al2O2, ZrO2, and MgF2, or multilayer film made of TiO2 and SiO2 on the principal surface of the infrared-absorbing glass. The infrared reflector is produced by forming an infrared-reflecting film on a principal surface of a transparent substrate. The infrared-reflecting film is a multilayer film composed of first thin films made of a high refractive index material such as TiO2 and second thin films made of a low refractive index material such as SiO2, the first and second thin films being alternately laminated.
Patent Literature 3 describes an optical film having a near-infrared-absorbing layer including a transparent resin containing a given organic dye.
Patent Literature 4 describes an optical filter including a near-infrared-absorbing layer containing at least one component selected from: a component composed of a given phosphoric acid ester compound and copper ion; and phosphoric acid ester-copper compound obtained by a reaction between a given phosphoric acid ester compound and copper compound. The near-infrared-absorbing layer may be formed from a resin composition including a resin such as an acrylic resin that contains the above component(s).